The present invention relates to an ultraviolet induced fluorescence monitor for detecting the presence and quantity of pollution and more particularly to a pollution monitor of this type having an in situ calibration capability. The present invention finds particular utility in detecting accurately the concentration of gaseous pollutants, such as sulfur dioxide (SO.sub.2) and nitric oxide (NO), in a sample stream or plume of ar, smoke-stack exhaust or other fluids.
As discussed in U.S. Pat. No. 3,795,812, granted Mar. 5, 1974, there are about eighty million tons of SO.sub.2 released into the air each year by the burning of sulfur-containing coal and oil and the refining of oil and various metals. As explained, SO.sub.2 concentrations of a few parts per million (ppm) of air can cause breathing difficulty, kill plants and hasten the deterioration of papers, leather and limestone building materials. It has therefore been proposed to limit the concentration of SO.sub.2 and other stack gases to a maximum of 500 ppm.
The SO.sub.2 pollution monitor described in U.S. Pat. No. 3,795,812 has a fluorescence cell into which is admitted a sample stream of air, smoke-stack gas or the like. As this sample flows through the cell, it is irradiated with ultraviolet energy in the region from about 2100 to 2300 angstroms (A) and fluoresces. This SO.sub.2 fluorescence lies in the band from about 2400 A to 4200 A, and the intensity of this fluorescence is measured at right angles to the incident beam. The response of the measurement is linear over a wide range of SO.sub.2 concentrations and is negligibly or only moderately affected by the presence of water vapor in the sample stream.
In the calibration of fluorescence monitors, a common technique is to flow a calibration gas of known concentration through the instrument and observe the output meter or recorder to ascertain if the instrument is reading correctly. If it is not, then the calibration controls, such as the gain and span controls associated with meters of this type, are adjusted until an accurate reading is obtained.
The problems inherent in this type of calibration is that the calibrating gas may not have the specific concentration stated because of the deterioration of gases such as SO.sub.2 and NO with time. Thus, if the calibration is made to an incorrect standard, an error is incorporated into the measuring apparatus and the measurements of pollutant concentrations which follow will be inaccurate. An additional disadvantage is where it is desired to recalibrate or standardize the monitor while in the process of making measurements. This requires disconnecting the monitor from the source of sample gases which are flowing through the instrument, and connecting the source of calibration gas so that the calibration can be performed. Such calibration technique is obviously time consuming and inefficient. There is additionally the inconvenience of obtaining and transporting bottled calibration gas including the necessary valves, regulators and other plumbing which must be used.